Conventionally, twisted nematic liquid crystal (hereafter referred to as “TN liquid crystal”) display elements and organic electroluminescence (hereafter referred to as “organic EL”) elements have been known to be thin, low power display elements that can be used in personal digital assistants. In TN liquid crystal display elements, the alignment state of liquid crystal molecules in the liquid crystal layer changes only during the period in which a voltage is applied, by which the transmissivity of light that transmits through the liquid crystal layer is controlled to perform a display operation. For this reason, operating power is constantly required for the display operation, and images cannot be displayed in a non-powered state. Likewise, organic EL elements utilize light emission caused in a state where electric current or voltage is applied to display images, and therefore, the image display is impossible in a non-powered state, as with the TN liquid crystal display elements.
In contrast, a display element has been proposed to date that has such a characteristic that it requires voltage or current only when rewriting display images, and an display image that has been written once can be retained until the image is rewritten even under a non-powered state. Since such an element, unlike the TN liquid crystal display elements or the organic EL elements, does not require electric power for retaining images, the use of such a display element for, for example, a display unit in personal digital assistants can achieve considerable reduction in the power consumption and reduction in the device size. In addition, by adopting a configuration in which the rewriting device is detachable from the display element, namely a display panel, it is possible to realize a small thickness, light-weight display element that does not require a driver circuit and has flexibility. Such an element is advantageous for portable devices.
Display systems in these elements can be broadly categorized into a system using micro-particles, a system utilizing an electrochemical or photochemical reaction of a solution or the like, and a reflected light controlling system using an electromechanical effect.
For one example of the element using the display system using micro-particles, a research has been made on a display element that performs a display operation by applying an electric field to a system in which charged colored micro-particles are dispersed in a colored solution filled between a pair of substrates provided with a pair of electrodes so that the particles travel (electrophoretic phenomenon) in the solution. One example of such a display element has a configuration in which a two-color display is performed by, of two kinds of particles, causing the particles having a negative polarity to travel to the positive electrode side and causing the particles having a positive polarity to travel to the negative electrode side. Another example of that configuration is such that colored particles in a colored solution are caused to travel according to their polarities so that when the particles travel to an observer side, the color of the particles are observed, whereas when the particles travel to the far side from the observer, the color of the colored solution is observed. Further another conceivable example of that configuration is such that a multi-color display is performed using two or more kinds of colored particles and colored solutions.
In addition, another example of the use of the foregoing electrophoresis principle is a technique in which electrodes are formed in the same surface of a substrate and a multi-color display is achieved by using a state where particles are gathered on the electrodes and a state where the particles are dispersed over the surface. For example, there is a display system in which a narrow-width, fine-wire electrode and a wide-width, plate-shaped electrode are formed on a surface of one of transparent substrates, and a multi-color display is achieved by controlling a state in which charged particles are gathered by adhering them to the fine-wire electrode and a state in which the charged particles are dispersed by adhering them to the plate-shaped electrode. Also, there is a display system called a twist ball system in which a multi-color display is achieved by rotating spherical or cylindrical particles that are colored in at least two colors by an electric field.
In the display element that performs display by causing the particles to travel in a solution, the traveling velocity of the particles is affected by the viscosity of the solution. Specifically, when the particles travel in a solution having a large viscosity, their traveling velocity becomes slow, and the display speed (response speed) of the element is accordingly slow. In addition, since there is no threshold value of the voltage at which the particles start to travel, an active matrix drive is necessary for the drive circuit. This increases cost.
In view of this, a system has been proposed in which particles are caused to travel in a gas phase, in which the traveling velocity of the particles is faster than in a solution. This system is such that at least one kind of charged colored particles are dispersed in a gas phase, and the particles are caused to travel between electrodes having opposite polarities by the Coulomb force of the electric field applied to the gas phase. In a gas phase, the traveling velocity of particles is fast because there is no such viscous drag of the traveling medium as in the liquid phase. As a result, the display speed becomes faster, and high-speed response is possible. Among such systems in which particles are caused to travel in a gas phase, there are a system having a configuration in which, using charged conductive toner particles and non-charged insulative particles, the charged particles are caused to travel by a Coulomb force (for example, see Japanese Unexamined Patent Publication No. 2000-347483), and a system having a configuration in which two kinds of particles having different polarities are caused to travel by a Coulomb force (for example, see Japanese Unexamined Patent Publication No. 2001-312225).
The display element that performs a display operation by causing particles to travel in a liquid phase or in a gas phase as described above requires a gap serving as the traveling space for the particles. The gap such as this is formed by supporting a pair of substrates opposed to each other by a spacer, which serves as a gap-retaining member.
FIG. 17 is a schematic view showing the configuration of such a display element. As shown in FIG. 17, in the display element, a substrate 51 and a substrate 52, on the inner surfaces of which an electrode 53 and an electrode 54 are formed, are supported by a spacer 56 and are opposed to each other, whereby a gap 55 is formed. In this gap 55, a plurality of negatively charged black particles 57 and a plurality of positively charged white particles 58 are contained, and the gap 55 serves as a traveling space 55′ for the colored particles. The traveling space 55′ is in a liquid phase or in a gas phase depending on the display system. In such a configuration, the traveling space 55′ for the colored particles is partitioned by the spacer 56, and accordingly, the spacer 56 serve as a partition wall in the space (hereafter, the spacer 56 is referred to as “partition wall 56′”).
Here, in the traveling space 55′, there occurs a phenomenon in which when the colored particles 57 and 58 having different polarities travel in the space toward respective electrodes 53 and 54 according to their polarities, the particles aggregate to one another since the particles having different polarities come into contact with one another a number of times. This causes luminance unevenness in each pixel of the display element. In a rewritable display element, the number of times the particles come into contact one another increases according to the number of rewriting operations since rewriting of the display is performed over and over again. Consequently, such an aggregation phenomenon becomes a cause of display image quality degradation. In particular, it is known that the traveling of particles in a gas phase is restrained mainly due to the contacting between particles having different charging characteristics. Therefore, the aggregation phenomenon occurs and the display image quality degrades when the flowability between particles having different charging characteristics is low.
Moreover, in the traveling space 55′ partitioned by the partition wall 56′, particles adhere to the partition wall surface or aggregate in the vicinity of the partition wall due to a force such as an image force. For this reason, luminance unevenness is observed in the partition wall vicinity. In addition, when the black particles 57 and the white particles 58 are contained and dispersed in the traveling space 55′ partitioned by the partition wall 56′, aggregation occurs between particles having different polarities due to an electrostatic force or between particles due to van der Waals force; therefore, dispersion of the particles becomes non-uniform, and uneveness is observed. Such dispersion uneveness of the particles causes luminance unevenness to occur.
Meanwhile, in view of the above-described problem, various techniques have been proposed for crushing the aggregated group of particles and dissociate them into individual particles. Among them, one effective technique is such that the vibration of ultrasonic wave or the like is used to dissociate aggregated particles. For example, a display element has been disclosed in which a vibration-imparting means is disposed at the reverse surface side (that is, the far side from the traveling space for particles) of a substrate (see, for example, Japanese Unexamined Patent Publication Nos. 2002-131789, H3-53224, and 2002-174828). The vibration-imparting means may have a configuration for imparting vibration to the display element itself, or a configuration for vibrating particles by an electric field or the like.
Nevertheless, the configuration of providing the vibration-imparting means for the display element separately is unfavorable in terms of cost. Further, for example, when the rigidity of the substrate is higher, the vibration generated by the vibration-imparting means is transmitted more from the substrate to the particles in the traveling space, and accordingly, as the rigidity increases, the amplitude of vibration must be increased accordingly. For this reason, the size of the vibration-imparting means also needs to be large, which requires a housing structure for preventing reduction in the vibrational energy from the vibration-imparting means. This increases the cost of the display element.
Moreover, the configuration in which a vibration-imparting means is provided requires a voltage to be applied to the vibration-imparting means for generating vibrations in addition to the image signal voltage applied for rewriting display images. Here, when the particles are caused to travel according to their polarities between the electrodes having opposite polarities, the particles cannot be detached and caused to travel from one of the electrodes to the other electrode unless the Coulomb force of the electric field generated by the application of an image signal voltage is greater than the adhesive force (specifically, the van der Waals force and the image force) between the one of the electrodes and the particles adhering thereto; therefore, a very high operating voltage is required to rewrite display images. Consequently, the display element that requires an operating voltage for the vibration-imparting means necessitates further higher operating voltages, and if the applied voltage is low, luminance unevenness occurs as well as contrast degradation.